Genetic variation for virulence and resistance in the wheat Mycosphaerella graminicola pathosystem .3. Comparative seedling and adult plant experiments

Fourteen Dutch Mycosphaerella graminicola isolates were studied for their virulence to 22 wheat cultivars in the seedling stage in an experiment set up as a completely randomized block design with three repetitions. Isolate x cultivar interactions were highly significant. Cluster analyses were applied to select three isolates with significantly different virulence characteristics for both disease parameters. These were retested in the seedling stage and used to inoculate two field experiments that were planted according to a split plot design in 1992 and 1995. Overhead inoculations were conducted after flowering to avoid the effects of plant height; hence, these experiments were intended as monocyclic tests for virulence differences between the isolates. Significant isolate x cultivar interactions were detected in each experiment, demonstrating specificity in the wheat-M. graminicola pathosystem in the adult plant stage under field conditions. The reproducibility of the adult plant data was high. Genetic differences among the isolates were additionally demonstrated by randomly amplified polymorphic DNA (RAPD) patterns, which also showed that no significant contamination of the inoculated plots with the natural M. graminicola population had occurred. Rank correlations between seedling and adult plant data were significant for M. graminicola isolates IPO323 and IPO290, but not for isolate IPO001. Hence, evaluation of resistance and virulence may require seedling, as well as adult plant, tests.